There are several power converter topologies that have been developed over the years, which are intended to improve the power density and switching efficiency of power converters. An emerging focus of new converter topologies is to provide a means to reduce or eliminate converter switching losses, while increasing the switching frequencies. Lower loss and higher switching frequency means more efficient converters, which can reduce the size and weight of converter components. Additionally, with the introduction of high speed composite semiconductor switches, such as metal oxide semiconductor field effect transistor (MOSFET) switches operated by pulse width modulation (PWM), recent forward and flyback topologies are now capable of operation at greatly increased switching frequencies, such as, for example, up to 1.0 MHz.
However, an increase in switching frequency can cause a corresponding increase in switching and component stress related losses, as well as increased electromagnetic interference (EMI), noise, and switching commutation problems, due to the rapid ON/OFF switching of the semiconductor switches at high voltage and/or high current levels. Moreover, modern electronic components are expected to perform multiple functions, in a small space, efficiently, and with few undesirable side effects. For instance, a modern voltage converter that provides for relatively high power density and high switching frequencies, should also include uncluttered circuit topologies, provide for isolation of the output or “load” voltage from the input or “source” voltage, and also provide for variable step-up or step-down voltage transformation.
In an effort to reduce or eliminate the switching losses and reduce EMI noise the use of “resonant” or “soft” switching techniques has been increasingly employed in the art. The application of resonant switching techniques to conventional power converter topologies offers many advantages for high density, and high frequency, to reduce or eliminate switching stress and reduce EMI. However, the complexity required to provide control to the power switches (illustrated below as S1 and S2), and the components associated with complex control, create a limited use in commercial applications.
Conventional Flyback Voltage Converter Topology
FIG. 1 illustrates a flyback type voltage converter 100. The converter 100 includes a transformer 102, a resistor 104, two capacitors 106 and 112, and two diodes 108 and 110. The resistor 104 and the capacitor 106 are coupled in parallel. One node of the parallel resistor 104 and the capacitor 106 is coupled to a first terminal of the primary winding of the transformer 102. An anode of the diode 108 is coupled to the primary turns of the transformer 102, and the cathode of the diode 108 is coupled to the other node of the parallel resistor 104 and capacitor 106. An input voltage VIN is coupled to a first terminal of the resistor 104 and to a ground terminal. An anode of the diode 110 is coupled to a first terminal of a secondary winding of the transformer 102. A cathode of the diode 110 is coupled to a first terminal of the capacitor 112, A second terminal of the capacitor 112 is coupled to the second terminal of the secondary winding of the transformer 102. A first terminal of a switching component 115 is coupled to a second terminal of the primary winding of the transformer 102 to provide ON and OFF input power cycles to the transformer 102. A second terminal of the switching component 115 is coupled to a sense resistor 117, which in turn is coupled to ground. A load 114 is typically coupled to the output of the converter 100, at the secondary turns of the transformer 102.
The Flyback topology has long been attractive because of its relative simplicity when compared with other topologies used in low power application. The flyback “transformer” serves the dual purpose of providing energy storage as well as converter isolation, theoretically minimizing the magnetic component count when compared with, for example, forward converter. A drawback to use of the flyback is the relatively high voltage and current stress suffered by the switching components. Additionally, high turn-off voltage (caused by the parasitic oscillation between transformer leakage inductance and switch capacitance) seen by the primary switch traditionally requires the use of a RCD 108,106,104. This parasitic oscillation is extremely rich in harmonics and pollutes the environment with EMI, and causes high switching losses from the switching components in the form of extra thermal dissipation. These switching losses are further described below in relation to FIG. 2.
Conventional Flyback Timing Diagram
Accordingly, the converter 100 is configured to receive the input voltage VIN across the primary turns of the transformer 102, and provide power through the secondary turns of the transformer 102, to a load represented by the resistor 114. Also shown in FIG. 1, current at the primary side of the transformer 102 is proportional to the current flowing through the sense resistor and is represented by IPRI, while current at the secondary side is shown by ISEC.
The flyback voltage converter 100 suffers from loss, noise, and other inefficient and/or undesirable effects during operation, For instance, FIG. 2 illustrates a diagram 200 of the voltage and current signal curves recorded during the operation of the flyback converter 100 of FIG. 1. As shown in FIG. 2, the diagram 200 includes signals for the input voltage VIN, and a drain to source voltage VDS, across the switch 115 and current IPRI through the switching component 115, at times t1 through t5. Also shown in this figure, the illustrated signal curves include noise effects and a saw tooth shape that result from the hard switching of the flyback converter 100. The harsh electronic noise effects from ringing are particularly dramatic about the hard switch times of the switching cycle. Further, as mentioned above, these undesirable effects become even more pronounced at the higher switching frequencies required by modern voltage converter applications.
In an effort to reduce or eliminate the switching losses and reduce EMI noise caused by high switching frequencies, “resonant” or “soft” switching techniques are increasingly being employed. Resonant switching techniques generally include an inductor-capacitor (LC) subcircuit in series with a semiconductor switch which, when turned ON, creates a resonating subcircuit within the converter. Further, timing the ON/OFF control cycles of the resonant switch to correspond with particular voltage and current conditions across respective converter components during the switching cycle allows for switching under zero voltage and/or zero current conditions. Zero voltage switching (ZVS) and/or zero current switching (ZCS) inherently reduces or eliminates many frequency related switching losses.
Several power converter topologies have been developed utilizing resonant switching techniques, such as, for example, U.S. Pat. No. 5,694,304 entitled “High Efficiency Resonant Switching Converters,” to Telefus, et al., (Telefus), which is hereby incorporated by reference; U.S. Pat. No. 5,057,986 entitled “Zero Voltage Resonant Transition Switching Power Converter,” to Henze, et al., (Henze), which is hereby incorporated by reference; U.S. Pat. No. 5,126,931 entitled “Fixed Frequency Single Ended Forward Converter Switching at Zero Voltage,” to Jitaru (Jitaru), which is hereby incorporated by reference; and U.S. Pat. No. 5,177,675 entitled “Zero Voltage, Zero Current, Resonant Converter,” to Archer, (Archer), which is hereby incorporated by reference.
In particular, Henze describes single ended DC-DC flyback topologies for operation at very high switching frequencies, such as 1.0 MHz or greater. In Henze, a plurality of pulse width modulated (PWM) switches are utilized to effect zero voltage resonant transition switching. Jitaru describes variations of known forward and/or flyback converter topologies employing zero voltage and/or zero current resonant techniques. Jitaru specifically describes a forward converter topology utilizing resonant switching techniques to operate at constant frequency. Archer describes zero voltage, and zero current, switching techniques in resonant flyback topologies utilizing a resonant transformer assembly inserted in parallel with either the primary or secondary winding of the main transformer.
The application of such resonant switching techniques to conventional power converter topologies offers many advantages for high density, high frequency converters, such as quasi sinusoidal current waveforms, reduced or eliminated switching stresses on the electrical components of the converter, reduced frequency dependent losses, and/or reduced EMI. However, energy losses incurred during control of zero voltage switching and/or zero current switching, and losses incurred during driving, and controlling the resonance means, are still problematic. For instance, some researchers have implemented an active clamp in conjunction with a resonant converter circuit to realize the benefits of high frequency switching, while reducing its many side effects. See, for example, the United States patent to Telefus, incorporated by reference above.